A DC-to-DC converter is a voltage regulator that converts an input direct current voltage to a regulated output direct current voltage. Among different types of voltage regulators, switching regulators are increasing in popularity because they offer higher power conversion efficiency and design flexibility. A switching regulator typically uses a power switch, an inductor, and a diode to transfer energy from input to output. A feedback and control mechanism is used to provide a regulated output voltage. The most commonly used feedback and control mechanism is pulse-width modulation (PWM).
FIG. 1 (Prior Art) is a block diagram of a DC-to-DC converter 100 that generates a regulated output voltage VOUT. DC-to-DC converter 100 includes a PWM controller 101, a power switch unit 102, a comparator 103, an error amplifier 104, an inductor 105, a voltage divider network 106 formed by resistors 121 and 122, an output capacitor 107, and a load 108. PWM controller 101 includes a sequential logic element (for instance, an SR-latch circuit) 110. Power switch unit 102 includes a switch device 111 and a current sense amplifier 112. During operation, a control signal 116 is provided to control the ON and OFF states of power switch unit 102. The control signal 116 is a square wave with an adjustable duty cycle that is controlled by PWM controller 101. If the output voltage VOUT is higher than a desired regulation voltage VREG, then the ON time of switch device 111 is decreased so that VOUT is also decreased. Similarly, if the output voltage VOUT is lower than VREG, then the ON time of switch device 111 is increased so that VOUT is also increased. The output voltage VOUT is thus regulated by adjusting the duty cycle of control signal 116.
As illustrated in FIG. 1, control signal 116 typically has a relatively high frequency (for instance, 1.6 MHZ) for the output voltage VOUT to be filtered and converted to a DC voltage. A significant amount of power is therefore used to turn switch device 111 on and off frequently. This large amount of power used for switching results in low efficiency (output power/total power consumption) of DC-to-DC converter 100. The efficiency of a DC-to-DC converter is further reduced when operating under a light load condition with low output current. Accordingly, various techniques have been utilized to reduce the power lost to switching and thereby to increase the efficiency of the DC-to-DC converter. In the example of FIG. 1, the switching power loss is reduced by decreasing the switching frequency of switch device 111 when DC-to-DC converter 100 operates under a light load condition. Other improvements are desirable to further reduce the overall power loss and thereby increase the efficiency of a DC-to-DC converter.